Biological Molecules

• consist of 1 or more polypeptides folded into globular/fibrous form
• A polypeptide is a linear polymer chain of amino acids bonded together by peptide bonds

• One amino acid loses a hydroxyl (-OH) group from (-COOH) group while the other loses a H atom from (-NH₂) group
• C atom of the first amino acid can thus bond with the N atom of the second, forming a peptide bond

• Simple linear strand of amino acids held together by peptide bonds in the polypeptide
• Based on the different arrangements of the 20 fundamental amino acids.
• Amino acid sequence det the biological function, furthest levels of organisation and hence different properties of proteins.
• Thus, a change in 1 amino acid may completely alter shape and properties of the polypeptide

• Polypeptide chain that is coiled up and held together by many intra-molecular H bonds
• H atom of NH group of 1 amino acid H-bonded to O atom of C=O group of another amino acid 4 places ahead
• H bond if the result of e^- sharing in the NH group, leaving H atom slightly +ve and the C=O group, leaving O atom slightly -ve, so they attract each other.

• 1 polypeptide chain consisting of parallel adj chains running in either opp or same direction
• Chains joined by H bonds between H atom of NH group of 1 amino acid and O atom of C=O group of another amino acid in adj chain
• Structure gives protein stability, high tensile strength & flexibility

• One polypeptide chain further coiled and extensively folded to form compact 3D globular structure.
• Bonds: disulfide bridges, ionic bonds, H bonds, hydrophilic/hydrophobic interactions between R groups
• Tertiary structure gives proteins their specific 3D configuration

• Aggregation of 2 or more extensively coiled polypeptide chains
• Bonds: disulfide linkages, ionic bonds, H bonds, hydrophobic/hydrophilic interactions between R groups

• Strong covalent bonds formed between adj amino acids
• Strongest bonds in polypeptide chains

• Strong covalent bond from from oxidation of sulphydryl (-SH) groups of 2 neighbouring cysteines' R groupds
• Broken by reducing agents (e.g. urea)

• Strong bonds formed between ionised/charged (-NH₂) & (-COOH) groups.
• R groups containing COO^- attracted to R groups with NH₃⁺
• Formed at suitable pH but can be broken if pH changes

Weak bonds formed when hydrophilic R groups project out and interact with water while hydrophobic R groups shielded inside protein interact with non-polar R groups of amino acids

• H atoms in NH groups or OH group form bond with O atoms in OH groups
• H bonds formed between R groups in tertiary structure
• Numerous; stabilise the structure

• Polypeptide chain first coiled into alpha-helices, then folded into spherical globular protein held by H bonds and ionic bonds
• Hydrophobic side-chains point inwards to maintain 3D shape. Hydrophilic side-chains point outwards to maintain solubility
• Each chain carries prosthetic haem group containing Fe²⁺ which binds to a molecule of oxygen.
• Thus each haemoglobin molecule can bind 4 O₂ molecules

• Tropocollagen consists of 3 polypeptide chains wound around each other to give triple helix
• Each polypeptide chain is a loosely wound left-handed helix that wind around 2 others and twist into a triple helix, linked by H bonds
• Glycine at every 3rd amino acid allows close packing to form tight coil for compact structure

• Covalent bonds formed between the lysines in adj chains. Cross-links hold tropocollagen molecules side by side, forming fibrils.
• In fibrils, tropocollagens lie parallel with staggered ends, overlapping aids in forming strong fibres.

• Synthesis of collagen polypeptide occus on the ribosome on RER
• Collagen threaded into RER lumen where loose winding of polypeptide and hydroxylation of selected prolines & lysines occur
• ER vesicles carrying polypeptide bud off ER, travel to GA where glycosylation of selected hydroxylysines occur
• Further assembly of 3 collgen polypeptides forms tropocollagen. Glycine molecule at every 3rd AA allows close packing of collagen molecule, which buds off GA, forming secretory vesicle
• Secretory vesicle moves towards and fuses with CSM
• Secreted into extracellular matrix
• Polypeptides cleaved to remove N & C terminals
• Self assembly of tropocollagen into fibrils via cross-linking between lysine residues and overlaps of staggered ends
• Aggregation of collagen fibrils form fibres.